For froth flotation, the ground ore is mixed with water to form a slurry and the desired mineral is rendered hydrophobic by the addition of a surfactant or a collector chemical, such as a depressant, although some mineral surfaces are naturally hydrophobic requiring little or no addition of collector. The particular chemical depends on the nature of the mineral to be recovered and, perhaps, the natures of those that are not wanted. As an example, sodium hydrosulfide (NaHS) may be added as a depressant in the selective flotation of molybdenum to separate it from copper. This slurry of hydrophobic particles and hydrophilic particles is then introduced to tanks known as flotation cells that are aerated to produce bubbles. The hydrophobic particles attach to the gas bubbles, which rise to the surface, forming a froth. The froth is removed from the cell, producing a concentrate of the target mineral.
The minerals that do not float into the froth are referred to as the flotation tailings or flotation tails. These tailings may also be subjected to further stages of flotation to recover the valuable particles that did not float the first time. This is known as scavenging. The final tailings after scavenging are normally pumped for disposal as mine fill or to tailings disposal facilities for long-term storage.
Flotation is normally undertaken in several stages to maximize the recovery of the target mineral or minerals and the concentration of those minerals in the concentrate, while minimizing the energy input.
Addition of flotation reagents is adjusted based on the pH and redox potential measured from the slurry. High content of oxygen in a flotation cell is known to increase the consumption of the flotation reagent and increase flotation costs. Therefore several methods for controlling the oxygen content of the utilized process gas, i.e. flotation gas, have been developed.
WO2004/080599 discloses a method for separating minerals from a slurry containing valuable minerals, wherein gases fed in different process steps, including flotation, are recirculated in an essentially closed gas circulation created around the equipment used. According to the publication, in flotation, the recirculation of the flotation gases allows more efficient optimization of the froth structure. In accordance with the method disclosed in the publication grinding, flotation, precipitation and filtering should all be performed in a fully sealed, controlled recirculating gas atmosphere.
Replacing air with a non-oxidizing inert gas in mineral separation has been proposed, for instance in U.S. Pat. Nos. 6,032,805, 6,036,025, 6,041,941 and U.S. Pat. No. 6,044,978.
In some certain flotation processes, in which it is required to use inert gas as flotation gas, there is an economic incentive to partially or fully capture and reuse the flotation gas, in order to reduce the consumption and thus the cost of the said inert gas (e.g. nitrogen). When using forced-air type flotation machines, the flotation cells are fed with gas which is pressurized with a compressor or blower. Therefore the recirculation system has to be able to collect the gas from the cells and redirect the gas back to the compressor or blower in a closed-loop piping arrangement i.e. a flotation gas recirculation loop.
In some cases, the flotation reagents used may cause unintentional formation of harmful byproduct gases (e.g. H2S). When such gases are formed, the gas collection and supplying system has to be hermetically sealed in order to keep the harmful byproduct gases contained. Also, the efficiency and economy of the system is better when the system allows less losses of inert flotation gas.
Thus, a conventional gas recirculation system is mechanically sealed. However, due to the variability of the volume of natural gases, the sealed flotation system has to have some means to compensate for volume changes. For this, special equipment called gas buffer tank is commonly used. It is able to store and release extra volume of flotation gas when needed. This may happen, for example, when the flotation blowers are shut down and the bubbles in the slurry are released into the recirculation piping, causing the internal total volume of gas in the system to expand. Also, when the outside temperature or ambient air pressure fluctuates, so has to happen also to the gas inside the system, or otherwise mechanical loading due to internal and external pressure differences are incurred to the flotation machines and piping.
Therefore the existing methods do not allow utilization of a closed system with controlled point of release for expulsed flotation system without a need for expensive and large gas buffer tank.